Oracle Blog

From My Brain to Your Browser

What's Pluto?

Recently a young person asked me why
Pluto isn't a planet.
This seemed like a good educational opportunity. The explanation I used is
much simpler than the official, scientific explanation - with its
"planetary discrimants"
and "aggregate masses" - and turned out better than I anticipated, so
I thought I would share it with you. It seems appropriate for people
with at least a fourth or fifth grade education.

rock-ice bodies - objects with significant amounts of rock and ice,
e.g. comets,
Pluto and its satellite Charon - their densities range from
1.0 to 3.0 g/cm\^3, with almost all of them in the range 1.0 to 2.0 g/cm\^3.

The clearest distinction is between the three densest inner objects
(Mercury, Venus, Earth) and all but one of the outer, rocky/icy objects.
Haumea,
in case you haven't heard of it, is the newest object to be
labeled a "dwarf
planet" by the International Astronomical Union.

Although there is overlap in density between the gaseous objects and the
rocky/icy objects, no overlap between their sizes (masses) exists, as
this next graph shows (Earth is arbitrarily assigned a value of 1,000,000
and the rest are scaled to that value):

Visually, three groupings are discernible: the gaseous objects, the
large rocky objects, and everything else. Mathematically, the groupings
are separated by more than an order of magnitude. In other words, the
smallest member of one group is at least ten times the mass of the largest
member of the next group. Uranus is more than 14 times the mass of Earth,
and Mercury is almost 20 times the mass of Eris, which is more massive than
Pluto.

Besides physical characteristics, the most useful ones are the orbital
elements.

All members of our solar system orbit the sun, or orbit another non-stellar
object which in turn orbits the sun. These orbits are, almost entirely,
described by Newtonian
mechanics, the basic elements of which were first
described by Johannes
Kepler in 1609. Although an orbit has several
characteristics, the simplest of them is the semi-major axis, which is
often called the
"average"
distance between the orbiting body and the sun. Although not quite accurate,
it's close enough for this purpose.

Here is a graph of 17 of the most important bodies in our solar system. It
shows the semi-major axis of each, relative to the semi-major axis of Earth,
which is called an Astronomical Unit.
It includes the four major rocky objects, the four gaseous objects, all
five of the currently recognized dwarf planets, three asteroids, and five
other relevant objects. Note that the orbital distances of Eris (68 AU)
and Sedna (526 AU) are off the scale of this graph.
Again, three groups appear in the graph: the inner rocky objects, the
gaseous objects, and the outer bodies. Separation between objects increases
from the inner bodies to the outer ones, but suddenly, starting with Orcus,
the separation between orbital distances shrinks considerably.

From those three characteristics: density, mass, and orbital distance,
it seems clear that there are at least three groups of major bodies in
the solar system:

inner, rocky bodies, each having a mean density greater than 3.8 g/cm\^3,
a mass larger than one-tenth Earth's mass, and an orbital distance less than
than 2 AU

giant gaseous objects, each with a mean density less than 1.5 gm/cm\^3, a
mass larger than 14 times Earth's mass, and an orbital distance between 5 and
32 AU

distant icy, rocky bodies, with a mean density less than 3 gm/cm\^3 (and
almost all less then 2), and an orbital distance greater than 35 AU.

Object Type

Densityg/cm\^3

Mass(Earth=1)

Semi-Major Axis(AU, Earth=1)

Inner, rocky

>3.8

>0.1

<2

Giant, gaseous

<1.5

>14

5-32

Distant, icy, rocky

<2 (except one)

<1/200th

>35

All three of those groupings can be displayed in one graph, which shows
the distinction between the three different groups at a glance:

In the graph above, the green bars show the range of values for the inner,
rocky bodies, scaled so that the highest value is 100. The blue bars
show the ranges of values for the gaseous bodies. The purplish bars
shows the ranges of values for the outer icy, rocky bodies.

Note that only two ranges overlap: densit for the gaseous and the icy rocky
bodies. For all of the other characteristics, there are clear gaps between
the ranges of the groups.

Now that we have clear groupings, the question becomes "which of those
groups should be planets?" I hope it's obvious that
the first two categories should be included in the list of 'planets.'
The third group can be included or not, depending on how you want to define
the term 'planet.'

However, there are two other factors which help me to decide. Initially,
'planets' were the five wandering lights that weren't the Sun and Moon.
These seven wandering lights were so important that early western cultures
assigned a day of worship to each, leading eventually to
the
names of our days.

If 'planets' started with the five wandering stars, it makes sense to
add other bodies which have similar characteristics - Uranus and Neptune -
yielding a total of eight planets. But none of the others - Pluto, Haumea, Quaoar,
etc. - are like the original wandering lights in the sky.

Further, if we were to include the outer, rocky, icy bodies in the list of
planets, the list grows significantly. Today, the list would include
13 members, but another 40 known objects might be categorized with Eris,
Pluto, et al., and another 150 or more are probably out there. If the category
'planet' can have 8 members or 200, I'll go with 8.

Finally, regarding the question "is it 'right' to 'demote' Pluto?" The list of planets
has grown and shrunk
several
times throughout history. More than 25 bodies have been labeled 'planets'
only to be 'demoted' later. Pluto is nothing special in this regard.

Seriously, if size is the only criterion for distinction as a planet, then you must decide on a threshold. It is difficult or impossible to do that without being arbitrary. Science doesn't like arbitrariness.